An actuator for a magnetic disc apparatus has a carriage supporting a magnetic head mounted thereon. By driving a voice coil motor (VCM), the carriage is reciprocally rotated to reciprocally rotate the head across a magnetic recording medium (or disc) so as to position the head at a desired track of the disc.
FIG. 11 shows this type of actuator. As shown, the carriage a has a plurality of arms b. A magnetic head d for recording and reproduction is mounted via a suspension on the free end of each arm b. A moving coil e of a VCM is mounted on the opposite side of the arms b of the carriage a. Permanent magnets f are mounted in an interval above and below the moving coil e. As shown in FIG. 10, the actuator is disposed aside a plurality of discs g disposed one above another in an internal, the magnetic heads provided on the end of the arms b are provisionally set over the discs g. The VCM is driven by passing the current to the moving coil e, the carriage a is reciprocally rotated in the directions of arrows X in FIGS. 10 and 11, thus causing reciprocal rotation of the arms b in the same directions across the discs g, which are being rotated by a motor (not shown). In this way, each magnetic head d is positioned to a desired track of each disc g.
In the prior art, carriages having various structures and shapes have been used. FIG. 8b shows one such carriage. This carriage is obtained by cutting an eventual carriage a having a shape as shown in FIG. 8A, which is an extrusion molding product w of aluminum having a angularly cylindrical body part h and an arm part for a plurality of arms b. This molding product w is cut to form a body h and the plurality of arms b as shown in FIG. 8B. For mounting the moving coil e of the VCM on the carriage a, the carriage a and the moving coil e are set in a resin molding die, and a resin is then pressure poured into the die. In this way, as shown in FIG. 8C, the outer periphery of the moving coil e is molded in the resin j and also molded by the same to the body h. It is possible to fabricate the eventual carriage a shown in FIG. 8b by aluminum die casting as well.
Where the eventual carriage shown in FIG. 8B is fabricated as an extrusion molding product, the following drawbacks are present.
1 Operations of cutting and burr removal, which should be done after the molding operation, are cumbersome and increase personal expenditures and cost.
2 Particularly, it is necessary in the burr removal operation to remove even very fine burrs of the order of microns. This operation, therefore, is thus difficult to be done with the naked eyes, and it is necessary to use an enlarging lens or a microscope, leading to inferior operation efficiency and increasing the personal expenditures.
3 Usually Series 6063 aluminum is used for the extrusion molding. This material has viscosity, making it difficult to remove burrs and also cut it. More cost is thus necessary than in the case of the aluminum die casting.
4 Mass production property is lacking, leading to cost increase.
5 High equipment cost for the machining is required, which is a great economical burden.
Where the eventual carriage shown in FIG. 8B is fabricated as an aluminum die cast product, the following drawbacks are present.
1 Operations of burr removal, cutting burr removal again and plating, which should be done after the molding operation, are very cumbersome and increase costs.
2 Particularly, it is necessary in the burr removal operation to remove even very fine burrs of the order of microns. This operation, therefore, is thus difficult to be done with the naked eyes, and it is necessary to use an enlarging lens or a microscope, leading to inferior operation efficiency and increasing the personal expenditures.
3 The product has numerous micropores. If the micropores are left as such, air that is heated by heat generated from the disc drive motor is gasified and blown against the disc g through the micropores, and the disc g may thus be fogged or contaminated or damaged.
4 To preclude the drawback in 3 above, the plating film may be formed as thick as being enough to completely close the micropores. By so doing, however, the cost is increased.
5 From the above drawbacks, a lower limit is imposed on the thickness of the arms, and the arms are inevitably thick and heavy.
FIGS. 9A to 9C show a molded carriage a, which is provided for precluding the above drawbacks. This carriage a is fabricated by simultaneously molding a body h and arms b produced with a press in resin j. Specifically, the carriage a is obtained by setting a plurality of aluminum arms b such as to be spaced apart at an interval in a resin molding die, and resin is pressure poured into the die, thus molding a resin body h while at the same time burying stem parts of the arms b in the resin of the body h and thus making the arms b to be integral therewith. When mounting the moving coil e on this carriage a, the carriage a and the moving coil e are set in the same resin molding die, and resin is again pressure poured thereinto, thus further molding the outer periphery of the preliminary resin molded body h in the resin while at the same time molding the moving coil e in the resin to be integral with the body h.
This molded carriage is free from the drawbacks inherent in the aluminum die cast carriage or the extrusion molding product carriage, but it ha the following drawbacks.
1 Since only end parts (i.e., stem parts) of the arms b are molded in the resin j when forming the body h by the resin molding, unlike the one-piece molding having the body h and the arms b, the end parts of the arms b are liable to be vibrated. The actuator is reciprocally rotated several tens of thousands of time per second, and it is thus vibrated, although slightly, as it is moved. These vibrations give rise to a resonance, thus increasing the vibrations of the end parts of the arms b. Usually, the magnetic head d mounted on the free end of the arm b as shown in FIG. 11 and the associated disc g as shown in FIG. 10 are spaced apart by as small distance as several microns. Therefore, by the resonance noted above the head d may be brought into contact with the disc g, and both the head d and the disc g may possibly be damaged.
2 Since the body h is made of a resin while the arms b are made of aluminum, the arms b may be expanded and distorted in the order of microns by heat generated from the motor driving the disc g. By such distortion, the head mounted on the end of the arm b may be brought into contact with the disc g.
3 Since the arms b are set such that they are spaced apart in the die for molding their stem parts in the resin h, deviation of their position by the pressure of the resin pressure poured into the die is liable, resulting in fluctuations of their mounting positions relative to the body h and also their interval. Particularly, deviation of the dimension, by which the arms b product from the body h, is liable, resulting in fluctuations of projecting length of the arms b.
4 A resin molder for molding the body h, a resin molder for molding the arms b and man power for setting the necessary number of arms b in the die for molding the moving coil e to the body h, are necessary, thus increasing the cost. Since a bearing insertion part of the carriage is made of resin, resonance generation is liable, prevention of this dictates a bearing assembly having a complicated structure.
5 In the carriage a, as shown in FIG. 10, a bearing (rotary bearing) p with a sleeve o is secured to the body h by pressure fitting it in a shaft hole m of the body h, and a shaft q is secured to the bearing p by pressure fitting it in a center hold of the bearing p, thus permitting smooth reciprocal rotation of the carriage a. However, since the sleeve o, which is fabricated from a metal with a high machining accuracy, is pressure fitted in the shaft hole m in the resin which is difficult to ensure high machining accuracy, it is difficult to make the shaft hole h and the sleeve o to be integral; thus readily giving rise to resonance.